Field of the Invention
This invention relates to a conductive rubber roller used in an image-forming apparatus, such as electrophotographic copying machines, electrophotographic printers, and electrostatic recording apparatus, wherein the conductive rubber roller is disposed in contact with an electrophotographic photosensitive member.
In the image-forming apparatus, such as electrophotographic copying machines, electrophotographic printers, and electrostatic recording apparatus, a toner used as a developer is made to adhere to an electrostatic latent image formed by exposing an electrophotographic photosensitive member, which has been electrostatically, uniformly charged, and then the toner (toner image) is transferred to a transfer medium such as paper to form an image. Also, methods for charging the electrophotographic photosensitive member include a noncontact charging method utilizing corona discharge and a contact charging method making use of a conductive roller. Transfer methods also similarly include a noncontact corona transfer method and a contact roller transfer method.
In the conductive roller, a conductive rubber material having an electrical resistance of from 1xc3x97105 to 1xc3x971011 xcexa9xc2x7cm as volume resistivity is used. Such a conductive rubber material is compounded with a conductive filler such as carbon black in order to achieve the intended conductivity. In the conductive rubber material thus obtained, however, its electrical resistance is influenced by changes in the applied voltage, and hence an applied-voltage control unit must be provided when used as a charging member. Also, such a conductive rubber material may have non-uniform resistance value depending on how the conductive filler stands dispersed in the rubber material, and it has been difficult to obtain rubber materials having stable electrical resistance.
As a means for solving such a problem, a method is known in which a conductive rubber which is a polymer having a low electrical resistance is used for a rubber component used as a charging member to attain a stated electrical resistance. The conductivity of such a conductive rubber material does not rely on a conductive filler, such as carbon black. Hence, it has a small variation in electrical resistance depending on material lots or a small dependence on the applied voltage, and is a material, which is very easy to handle. It, however, has a disadvantage that it has a great difference in electrical resistance between a low-temperature, low-humidity environment and a high-temperature, high-humidity environment, in other words, a high environmental dependence.
Because of an advantage of a relatively small variation of electrical resistance, conductive rubbers, such as an acrylonitrile butadiene rubber and an epichlorohydrin rubber are used as materials for conductive rollers. Of these materials, the acrylonitrile butadiene rubber has a low resistivity of from 1xc3x97109 to 1010 xcexa9xc2x7cm and is inexpensive. Accordingly, it is in wide use as a material for conductive rollers, in particular, as a material for transfer rollers.
However, for reasons of making machinery compact and achieving cost reduction, power sources for applying electric charges to transfer rollers are also made compact, and have come to be of a type to which a great voltage cannot be applied. Accordingly, the rubber materials are also demanded to be those having a volume resistivity of from 1xc3x97108 xcexa9xc2x7cm to 1xc3x97109 xcexa9xc2x7cm, which is lower by about one figure than ever, and also those having a low environmental dependence.
An acrylonitrile butadiene rubber is commonly used in an acrylonitrile content ranging between 15% by weight and 50% by weight. However, in the case when the acrylonitrile content is in such a proportion, the electrical resistance is not so greatly variable, and the electrical resistance can only be regulated by a small amount.
As a method of regulating its electrical resistance, carbon black may be added. The addition of carbon black, however, is not preferable because it tends to cause a variation in electrical resistance.
Methods are also proposed in which an acrylonitrile butadiene rubber is blended with an epichlorohydrin rubber, which is also conductive-rubber. The epichlorohydrin rubber includes a homopolymer of epichiorohydrin and its copolymer with ethylene oxide. The product obtained by copolymerization with ethylene oxide has a lower electrical resistance because ethylene oxide is in a higher content in its composition. In proposals using a blend with an epichiorohydrin rubber, a blend proportion of the epichlorohydrin rubber is 25 parts or more based on 100 parts of the total weight, which is so high as to result in a great environmental dependence (Japanese Patent Application Laid-Open No. 8-292640) or, since a blend having a low ethylene oxide content of 40 mol % or less, i.e., one having a high electrical resistance is used, the electrical resistance can be regulated to be in a narrow range (Japanese Patent Application Laid-Open No. 11-65269), either of which is not preferable in the sense that the electrical resistance should be regulated to be at a low-resistance side, which is lower by about one figure.
An object of the present invention is to provide a conductive rubber roller which can solve the above problems, having a low environment dependence of electrical resistance and a small scattering of electrical resistance.
To achieve the above object, the present invention provides a conductive rubber roller comprising a conductive support and a rubber layer;
the rubber layer comprising a component (A), an epichlorohydrin rubber containing 40 mol % or more of ethylene oxide and a component (B), an acrylonitrile butadiene rubber component having an acrylonitrile content of 20% by weight or less.
the component (A) being in a proportion of 5 or more to less than 25 in weight ratio, based on the total weight of the components (A) and (B).
The present inventors have taken note of the fact that the electrical resistance of an epichlorohydrin rubber changes depending on an ethylene oxide content in the epichlorohydrin rubber and the electrical resistance decreases with an increase in the ethylene oxide content. They have considered that, an acrylonitrile butadiene rubber having a small environmental dependence should be blended with a small quantity of an epichlorohydrin rubber, in particular, one having a large ethylene oxide content, i.e., one having a low electrical resistance, whereby the resultant blend could be made to have a low electrical resistance while keeping small the environmental dependence of acrylonitrile butadiene rubber.
The conductive rubber roller of the present invention is described below in detail.
The conductive rubber roller of the present invention consists basically of a conductive support and a rubber layer.
As the conductive support used, any support may be used as long as it is electrically conductive and can withstand the load applied to the roller, such as rotation. Commonly used is a roller comprised of a metal such as iron or stainless steel, or any of these which has been plated.
Rubber components used in the present invention are an acrylonitrile butadiene rubber and an epichlorohydrin rubber. The acrylonitrile butadiene rubber and the epichlorohydrin rubber are highly compatible with each other, and become uniformly mixed when blended. As a result, the blended mix can be a rubber material having a small variation in electrical resistance.
The component (B), acrylonitrile butadiene rubber, is one having an acrylonitrile content of 20% by weight or less, and may preferably be one having an acrylonitrile content of 18% or less, and preferably 10% by weight or more as the lower limit. If the acrylonitrile butadiene rubber has an acrylonitrile content more than 20% by weight, it may have a high environmental dependence. If on the other hand the acrylonitrile butadiene rubber has an acrylonitrile content less than 10% by weight, it tends to have a high electrical resistance.
The component (A), epichlorohydrin rubber, is one having an ethylene oxide content of 40 mol % or more, and may preferably be one having an ethylene oxide content of 48 mol % or more, and preferably 65 mol % or less as the upper limit. This ethylene oxide content may be an ethylene oxide content, which is 40 mol % or more in the polymer composition, or may be so regulated by blending a plurality of epichlorohydrin rubbers having different ethylene oxide contents. The electrical resistance of the epichlorohydrin rubber becomes lower with an increase in the ethylene oxide content. If an epichlorohydrin rubber having an ethylene oxide content less than 40 mol % is used, the epichlorohydrin rubber must be blended in a large quantity in the acrylonitrile butadiene rubber used to attain a stated electrical resistance, resulting in a high environmental dependence. If on the other hand one having an ethylene oxide content more than 65 mol % is used, the ethylene oxide tends to crystallize to make the blend have both high electrical resistance and high environmental dependence.
The component (A) is in a proportion of 5 or more to less than 25, and preferably from 10 to 20, in weight ratio, based on the total weight of the components (A) and (B); the proportion being the value obtained by dividing the amount of component (A) by the total sum of those of the components (A) and (B), and multiplying the resultant value by 100 [(A)/{(A)+(B)xc3x97100]. If the epichlorohydrin rubber is present in an amount, which is less than this proportion, a reduced effect of lowering electrical resistance may be obtained. If it is more than that, a high environmental dependence may result.
In the present invention, the conductive rubber material is obtained by adding additives to the rubbers exemplified above, and dispersing them by kneading, followed by heating at 160 to 180xc2x0 C. for 10 to 50 minutes to effect vulcanization. As the additives, usable are conventionally known additives such as a vulcanizing agent, a vulcanizing accelerator, a softening agent, a plasticizer, a reinforcing agent, a filler and a blowing agent.
The conductive rubber roller of the present invention is commonly produced by extruding the above-mentioned conductive rubber material in a tubular shape, which is then subjected to vapor vulcanization, and thereafter a conductive support is press-fitted to the tubular product, followed by grinding to have a stated outer diameter. Various methods known conventionally may also be used, such as simultaneous extrusion together with the support and press vulcanization. The conductive rubber roller of the present invention may also be provided with a layer of resin or the like on a periphery of the rubber layer.
The rubber material used in the conductive rubber roller of the present invention may preferably have an electrical resistance of 1xc3x97108 xcexa9xc2x7cm or below, and particularly 1xc3x97108 xcexa9xc2x7cm or above, as a calculated volume resistivity in an environment of 23xc2x0 C./55% RH (N/N). Also, the conductive rubber roller of the present invention may preferably have an electrical resistance of 2xc3x97108 xcexa9 or below, and particularly 1xc3x97107 xcexa9 or above, as a resistance in an environment of 23xc2x0 C./55% RH (N/N).
The present invention is described below in greater detail by giving more specific constructions as Examples. The present invention is by no means limited to the scope exemplified below.